1. Field of the Invention
The present invention relates to a thin-film transistor which comprises a substrate, a gate electrode formed on the substrate, a gate insulation layer formed on the gate electrode, and a semiconductor layer formed on the gate insulation layer. The thin-film transistor is designed for use in, for example, an active matrix liquid-crystal display.
2. Description of the Prior Art
Attention is being paid to a thin-film transistor (TFT) which has amorphous silicon (a-Si) film and can be used as a switching element for an active matrix liquid-crystal display. FIG. 1 is a circuit diagram showing an active matrix liquid-crystal display having TFTs.
The display has many pixels arranged in rows and columns, one of which is enclosed within a circle in FIG. 1. Each pixel comprises a TFT 11, a liquid-crystal cell 12, an electrode 13, and a capacitor 14. The display has address lines 15.sub.l to 15.sub.m and data lines 16.sub.l to 16.sub.n. The gates of the TFTs of each row are coupled to the same address line. The sources or drains of the TFTs of each column are connected to the same data line. The use of the TFTs enables the display apparatus to display images of higher quality than an ordinary matrix liquid-crystal display which has no such switching elements.
Each TET of the active matrix liquid-crystal display shown in FIG. 1 includes an a-Si layer formed on a glass substrate. Since an a Si-layer can be formed at low temperature, an array of TFTs can be formed on a single glass substrate, in such a matrix pattern as is shown in FIG. 1. The use of a-Si and a glass substrate makes it possible to manufacture planar liquid-crystal display which is large, highly precise, and capable of displaying high-quality images.
As can be understood from FIG. 1, the address lines 15.sub.l to 15.sub.m must be both long and slender in order to provide a large, high-quality active matrix liquid-crystal display. Hence, it is required that these lines be made of greatly conductive material to have a relatively low resistance. In particular, it is difficult to satisfy the requirement for low resistance of the address lines, i.e., the gate electrodes of the TFTs when the TFTs are of the so-called inverted staggered type which is made by forming the gate electrodes on a glass substrate, then forming a gate insulation layer on the address lines, and finally forming a-Si film on the gate insulation layer. The reason of the difficulty is that the gate electrodes which is extremely thin must satisfy another requirement for resistance to the chemicals applied during the manufacture of the active matrix liquid-crystal display as well as the requirement for the high conductivity.
Film of a metal such tantalum (Ta) has been used as material of such gate electrodes. To manufacture a larger, high-quality active matrix liquid-crystal display, there is a great demand for other material which has a lower resistivity, a great workability, and a higher resistance to chemicals.
The more densely the display elements are arranged, the greater the total area of the intersections of the address lines 15.sub.l to 15.sub.m and data lines 16.sub.l to 16.sub.n Therefore, the insulation film interposed between the liens 15.sub.l to 15.sub.m, on the one hand, and the lines 16.sub.l to 16.sub.n, on the other, must have a great insulating strength. This inter-layer insulation film is formed at the same time, and of the same insulative material, as the gate insulation film. Hence, the gate insulation film must also have an adequate insulating strength and must provide good interface characteristics between the insulation film and the a-Si film formed thereon.
In order to provide a large, active matrix liquid-crystal display which includes a-Si film and display elements densely arranged in rows and columns, it is absolutely necessary to reduce the resistance of the gate electrodes of the TFTs incorporated in the display elements, and to use gate insulation film which has a great insulating strength and provides good interface characteristics along with the a-Si film.